# How do you compute Hess's law calculations?

##### 1 Answer

You have to develop a strategy for the order in which you add the various equations.

#### Explanation:

Hess's law states that the total enthalpy change does not rely on the path taken from beginning to end.

So, you can calculate the enthalpy as the sum of several small steps.

There are a few rules that you must follow when manipulating an equation.

- You can reverse the equation. This will change the sign of
#ΔH# . - You can multiply the equation by a constant. You must then multiply the value of
#ΔH# by the same constant. - You can use any combination of the first two rules.

**EXAMPLE:**

What is the value for the heat of combustion,

Given:

**Solution:**

Write down the target equation (the one you are trying to get).

Write down the three equations you must use to get the target equation.

Now we need to organize the given equations so that they add up to give the target equation.

A good place to start is to find one of the equations that contains the first compound in the target equation (

That would be equation 3, but we must reverse equation 3 and its

This equation contains

We have to eliminate these one at a time. First, we find an equation that contains

That would be equation 1, since we have already used equation 3.

At this point, we have

Now we work on the

We will use equation 2, but we will have to double it and its

We then get

Finally, we add the three equations to get the target equation, cancelling things that appear on opposite sides of the reaction arrows.

**Answer:**

The heat of combustion for the reaction is -1075.0 kJ.